The present invention relates to a radio communication system which includes a line controller, base stations, and mobile stations, and more particularly, to a radio communication system which has an improved architecture for assignment of a communication channel to a mobile station.
A standard has been established for a multi-channel access (MCA) radio system for airports which play an important role of ensuring the security of aircrafts existing in airports and the comfort of users. For example, see a document entitled “AIRPORT DIGITAL MOBILE TELECOMMUNICATION SYSTEM,” ARIB STD T-87, October 2003 by Association of Radio Industries and Business (ARIB).
In a system conforming to the foregoing standard, an airport, which constitutes a basic unit, is composed of control station facilities, base station facilities, and mobile station facilities, wherein communications are made between a domestic terminal and a mobile station or between mobile stations (through a base station or directly) using a 400 MHz band. The system employs a quadruple TDM/TDMA scheme which separates a single radio carrier on a frame-by-frame basis and groups the frames into four slots for providing multiple channels. The system also employs a n/4 shifted QPSK modulation scheme. Specifically, a base station is assigned a single or a plurality of radio carriers (pair waves) from a 400 MHz band which is an available frequency band. Each of a downlink radio carrier f1 and an uplink radio carrier f2 is divided into frames which are sub-divided into slots which are then allocated to a control channel and a communication channel.
For special requirements of operational communications within an airport such as consistent security for aircraft, this system is also required to permit ensured emergency communications and preferential connections for crucial or important communications.
For example, in a communication system which employs a multi-channel access (MCA) scheme as described above, a plurality of communication channels are commonly used by a multiplicity of users.
In such a multi-channel access based communication system, a base station, in response to a connection request from a mobile station, designates a communication channel assigned from a line controller for the mobile station, thereby permitting the mobile station to make communications through the designated communication channel.
In this event, upon receipt of the connection request from the mobile station, the base station requests the line controller to assign a communication channel, thereby acquiring information on the communication channel to be designated for the mobile station, from the line controller.
JP-A-11-262044 discloses a communication channel assignment method which enables a quick response to an assignment request for one or a plurality of channels from a mobile station in a multi-carrier TDMA base station which selectively combines a plurality of carrier frequencies with a plurality of time slots formed in time division at each carrier frequency to establish a plurality of radio communication channels. This prior art system does not have a line controller, but the base station contains a management table which is registered with one or a plurality of idle channels selected from a plurality of communication channels as candidate channels. Upon receipt of a channel assignment request from a mobile station, the base station references the management table to select a communication channel which satisfies particular communication quality conditions from the candidate channels for assignment to the requesting mobile station. The level of a received radiowave on each channel is relied on to determine whether or not the channel is idle. Then, a candidate channel, determined to be an idle channel based on the level of a radiowave received thereon, is used to communicate with at least one mobile station, thereby checking whether or not this is an effective candidate channel.